TweetFollow Us on Twitter

January 92 - The Soup Kitchen - Extending C++ with C++

The Soup Kitchen - Extending C++ with C++

Eric M. Berdahl

This month I'll look at C++ from a slightly different perspective. One of my central themes has been object code compatibility with Pascal and MacApp. But this will not be an issue today. Instead, I'll be looking at tools intended to make the C++ programmer's life easier by extending the language with "pure" C++ classes.

THE MEANING OF "EXTENDABLE"

Most languages are judged, at least initially, by the support they provide for certain "key" issues. String manipulation is a typical example of such an issue.

Support for strings in C and Pascal marks two extremes of a spectrum. Pascal provides rather intuitive syntax for dealing with strings, while C provides for support via a powerful but non-intuitive library of functions. The advantage of standard C is the programmer's ability to easily create superior string handling functions at the cost of readability. The advantage of standard Pascal is the programmer's ability to create very readable code at the expense of flexibility.

C++ provides something between these C and Pascal. C++ doesn't have built-in support for strings, but the class structure and messaging protocol allows you to design string classes that are intuitive and easy to change. When this is done, you can use strings as if C++ has predefined definitions for what strings are and how to manipulate them. Furthermore, C++ lets you create small classes whose sole purpose is to aid in housekeeping chores or other code-level cleanup that is sometimes required when programming complex systems like the Macintosh.

HOUSEKEEPING À LA C++

General techniques One of the more tedious tasks facing Macintosh programmers is the necessity to save different states at various points in one's code. MacApp does nothing but add to the complexity of the problem. Lock state of handles, pen states, focused views, and permanent allocation flags are simple examples of things MacApp programmers need to keep track of and keep in sync with code.

The classic approach to this problem is to write code resembling the following:

{
    StateRecord aState;
    GetCurrentState(aState);
    //  Insert code here that screws around
    //  with the element we want to save
    SetCurrentState(aState);
}

This code is straightforward and works fine. However, it will run into problems when you need to contend with MacApp failure handling. In this situation, you need to provide failure handling code to restore the state appropriately, as follows:

{
    StateRecord         aState;
    FailInfo            fi;
    GetCurrentState(aState);
    if (fi.Try())
    {
        //  Insert code here that screws around
        //  with the element we want to save
        SetCurrentState(aState);
        fi.Success();
    }
    else
    {
        SetCurrentState(aState);
        fi.ReSignal();
    }
}

This code is functional and perfectly acceptable, though rather wordy. It uses a simple concept-it preserves the state across this section of code. This bit of housekeeping is a perfect candidate for an object.

The technique I describe here applies to all forms of housekeeping. But for sake of brevity, I'll restrict my explication to one application-locking a handle.

CHandleLocker
class CHandleLocker
{
private:
    Handle      fHandle;
    Boolean     fWasLocked;
    FailInfo    fFailInfo;
protected:
    pascal void HandleFailure(OSErr err, long message);
    virtual pascal RestoreLockState();
public:
    CHandleLocker(Handle theHandle);
    virtual ~CHandleLocker();
};

CHandleLocker is a simple class that assures a handle remains locked within a particular scope and that it reverts to its previous state outside that scope. Using CHandleLocker, you can write code that utilizes handle locking in the following manner:

{
    CHandleLocker   lockSomeHandle(someHandle);
    //  Insert code that relies on someHandle being locked
}

Magically, when this code block executes, someHandle will be locked. The intervening code will execute, and someHandle will be returned to the state it was in (either locked or unlocked) when the block began. As an added bonus, if a MacApp failure is raised somewhere in the intervening code, someHandle will still be left in its original state. Amazingly, CHandleLocker adds safe handle locking to C++, and gives it a syntax that is relatively simple and unobtrusive.

Under the hood of CHandleLocker

To accomplish this little bit of heaven, CHandleLocker relies on the constructor and destructor mechanisms of C++ classes. The constructor and destructor are the core of the class. The constructor is called automatically when an instance is created, as in the declaration of lockSomeHandle in the example above. The destructor is called when an instance is destroyed, as in the closing curly brace in the same example.
CHandleLocker::CHandleLocker(Handle theHandle)
{
    fHandle = theHandle;
    fWasLocked = IsHandleLocked(fHandle);
    if (!fWasLocked)
        HLock(fHandle);
    CatchFailures(fFailInfo, CHandleLocker::HandleFailure, this);
}

CHandleLocker::~CHandleLocker()
{
    Success(fFailInfo);
    this->RestoreLockState();
}

The constructor simply saves the handle and its original lock state in instance variables. If the object is originally unlocked, it locks it. Finally, a MacApp failure handler is installed. In the destructor, the failure handler is removed and the lock state is restored. The remaining parts of the class are the method to restore the lock state and the failure handler:

void CHandleLocker::RestoreLockState()
{
    if (!fWasLocked)
        HUnlock(fWasLocked);
}

pascal void CHandleLocker::HandleFailure(OSErr err, long message)
{
    this->RestoreLockState();
}

RestoreLockState is straightforward. It simply refers to the fWasLocked boolean instance variable and unlocks the handle if it was originally unlocked. But the failure handler is a bit more involved and will take a short explanation.

CHandleLocker::HandleFailure is mediated using the Pascal-style failure handling routines, CatchFailures and Success. Therefore, the failure handling routine does not need to resignal the failure since returning from the routine automatically invokes the next failure handler on the stack.

Also, the method is declared with the keyword pascal, indicating that we want the return values and arguments passed Pascal-style. This is important once you recognize that methods with Pascal-style arguments will have the metavariable this passed as the last "argument" of the routine, the same position the Pascal static link variables occupy (see the September installment of The Soup Kitchen for a discussion of nested procedures and static links). Notice that this is passed as the static link variable in the CatchFailures argument list above.

That's all there is to this simple little utility class. I challenge the reader to write utility objects to handle other housekeeping chores. To get started, look at the PenState of QuickDraw, and the focus information in the MacApp view architecture. It should be fairly simple to use CHandleLocker as a template to build a useful library of magic little objects.

STRINGS

Admittedly, utility classes are simple examples and do not come close to making the case for language extensibility. However, having opened the door and provided some decent groundwork, let's take a look at CStr255, a very nice piece of work from the MacApp Team at Apple.

Any Macintosh programmer is familiar with the basic Pascal string. The Pascal string differs from the C string in the following ways:

  • Pascal strings are limited to 255 characters; C strings are basically unlimited in length.
  • Pascal strings contain the length of the string in their first byte; C strings are terminated with a null byte (hex 00).
  • Pascal strings are supported with Pascal language syntax; C strings are supported by convention and a standard C library.

Pascal programmers are fortunate to be able to use standard Macintosh strings, Str255, with little or no effort. Since these strings are nothing more than real Pascal strings, the compiler and run time environment take care of the details. The C++ programmer has the ability to use Pascal-like syntax to define and manipulate Pascal strings, but must be constantly aware of the difference and must maintain discipline to convert to the appropriate format as necessary.

Unlike the example of maintaining the lock state of a handle, writing a fully capable language-extending class like CStr255 is beyond the skill of all but master C++ architects. CStr255 provides an excellent example of what C++ can do, but it should not be imitated by the novice. This warning is sorely lacking in all C++ books I've encountered and is the primary cause of problems with novice and intermediate level C++ programmers. To quote Barnum and Bailey, "Kids, don't try this at home."

In its final form, CStr255 allows you to write code that looks like this:

{
    CStr255 aString;
    aString = "A string";
    aString = aString + "another string";
    if (aString == anotherString)
        gApplication->Beep(3);
    theLength = aString.Length();
    theFifthCharacter = aString[5];
    DrawString(aString);
    DrawString("A string constant");
    printf("This line prints C strings %s", (char*)aString);
}

In short, you can write code that looks like Pascal but that integrates well with your C++ code.

Constructor magic

The first rabbit I'll pull out of CStr255 involves its constructors. The designers of CStr255 provide a constructor that takes a single char* as an argument: CStr255::CStr255(char*).

Since all standard C strings and string constants in C++ are of type char*, this means you can assign C strings and string constants directly to any variable of type CStr255. Furthermore, any routine that takes a CStr255 as an argument, like DrawString, can be passed a C string or string constant with no difficulty.

Similarly, CStr255 provides for constructors from other Macintosh string types (CStr63 for example), other C types (like unsigned char*), and even a constructor from a longint that is used to convert ResType and OSType variables to their string equivalents. As with all the CStr255 code I'll quote from here, these constructors are found in the file PascalString.h in the MacApp 3.0b3 release.

Operator magic

Constructors are handy and useful, but without the operators +, =, ==, and others, CStr255 would just be a neat utility class. Instead, with these operators, you have access to a truly powerful class that adds Pascal strings to the C++ language with a very simple and intuitive syntax. Since it's not reasonable to reproduce the CStr255 code here, I'll leave you with a taste of the features available for string handling and short examples of their utility.

Constructors

CStr255();                            //  CStr255 aStr
CStr255(char*);                       //  aStr = "A string"
CStr255(const CStr255&);          // aStr = anotherStr255
CStr255(const CStr63&);           //  aStr = aStr63
CStr255(long);                        //  aStr = 'PICT'

Operators

char& operator[](short pos);      //  aStr[4] = 'P'
CStr255 operator+(CStr255&);      //  aStr = bStr + cStr
CStr255 operator=(CStr255&);      //  aStr = bStr
Boolean operator==(CStr255&);     //  if (aStr = bStr)
Boolean operator==(const char*);      //  if (aStr == "A string")
Boolean operator>(const char*);    //  if (aStr > "ABC")

Handy functions

char& Length();                   //  aStr.Length() = 7
Boolean IsEmpty();                    //  if (aStr.IsEmpty())

Conversions

operator char*();                     //  printf("%s", aStr);
operator long();                      //  GetResource(long(aStr), kID);
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

WhiteCap 6.6 - Visual plug-in for iTunes...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
VOX 2.8.14 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Paparazzi! 1.0b2 - Make user-defined siz...
Paparazzi! is a small utility for OS X that makes screenshots of webpages. This very simple tool takes screenshots of websites which do not fit on one screen. You specify the desired width, minimal... Read more
Carbon Copy Cloner 4.1.13 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
TrailRunner 3.8.832 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more
VueScan 9.5.65 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Adobe Illustrator CC 2017 21.0.2 - Profe...
Illustrator CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Illustrator customer). Adobe Illustrator CC 2017 is the industry... Read more
iDefrag 5.1.7 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
Quicken 4.4.2 - Complete personal financ...
Quicken makes managing your money easier than ever. Whether paying bills, upgrading from Windows, enjoying more reliable downloads, or getting expert product help, Quicken's new and improved features... Read more
iDefrag 5.1.7 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more

5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »
Last week on PocketGamer
Welcome to a weekly feature looking back on the past seven days of coverage on our sister website, PocketGamer. It’s taken a while for 2017 to really get going, at least when it comes to the world of portable gaming. Thank goodness, then, for... | Read more »
ROME: Total War - Barbarian Invasion set...
To the delight of mobile strategy fans, Feral Interactive released ROME: Total War just a few months ago. Now the game's expansion, Barbarian Invasion is marching onto iPads as a standalone release. [Read more] | Read more »
Yuri (Games)
Yuri 1.0 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: It's night. Yuri opens his eyes. He wakes up in a strange forest.The small, courageous explorer rides on his bed on casters in this... | Read more »
Space schmup Xenoraid launches on the Ap...
10Tons Xenoraid is out today on the App Store, bringing some high-speed space action to your mobile gadgets just in time for the weekend. The company's last premium title, another sci-fi game titled Neon Chrome, did quite well for itself, so... | Read more »
Star Wars: Force Arena Beginner's G...
Star Wars: Force Arena joined the populous ranks of Star Wars games on mobile today. It's a two-lane MOBA starring many familiar faces from George Lucas's famed sci-fi franchise. As with most games of this nature, Force Arena can be a little obtuse... | Read more »
Mysterium: The Board Game (Games)
Mysterium: The Board Game 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: The official adaptation of the famous board game Mysterium! | Read more »
Sonny (Games)
Sonny 1.0.4 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.4 (iTunes) Description: Reimagined for iOS, cult-hit RPG Sonny brings challenging turn-based combat that requires strategy and mastery of each new skill to... | Read more »
Towaga (Games)
Towaga 1.0 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: "It has been foretold that a masked being would stand atop the legendary Towaga Temple, dwelling among shadows to fulfil The Black Moon... | Read more »
Bubble Witch 3 Saga Guide: How to get th...
King's bringing its fairytale bubble-popping puzzler back for its 3rd outing in Bubble Witch 3 Saga. If you're familiar with the series, not much has changed here on the surface level, though you'll likely be pleased with the improvements. If you'... | Read more »

Price Scanner via MacPrices.net

Opera Announces Neon Concept Browser For Mac
Opera is inviting users to get a glimpse of what Opera for computers could become with its Opera Neon browser concept. Each Opera Neon feature is described as “an alternate reality” for the Opera... Read more
Tellini Releases TabView 3.0 Missing Tool fo...
Tellini has announced the release of TabView 3.0. TabView has been the first macOS viewer for PowerTab tablatures. PowerTab is a well-known and widely adopted tablature editor for Windows systems and... Read more
13-inch 1.6GHz/128GB MacBook Air on sale for...
Overstock.com has the 1.6GHz/128GB 13″ MacBook Air on sale for $130 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (MMGF2LL/A): $869.99 $130 off MSRP Their price is the lowest... Read more
12-inch 32GB Space Gray iPad Pro on sale for...
B&H Photo has 12″ Space Gray 32GB WiFi Apple iPad Pros on sale for $55 off MSRP including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $744.44 $55 off... Read more
9-inch 32GB Space Gray iPad Pro on sale for $...
B&H Photo has the 9.7″ 32GB Space Gray Apple iPad Pro on sale for $549 for a limited time. Shipping is free, and B&H charges NY sales tax only. Read more
Apple iMacs on sale for up to $120 off MSRP,...
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $120 off MSRP, each including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2199 $100 off MSRP - 27″ 3.2GHz/1TB Fusion iMac... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more
1.4GHz Mac mini, refurbished, available for $...
The Apple Store has Apple Certified Refurbished 1.4GHz Mac minis available for $419. Apple’s one-year warranty is included, and shipping is free. Their price is $80 off MSRP, and it’s the lowest... Read more
16GB iPad Air 2, Apple refurbished, available...
Apple has Certified Refurbished 16GB iPad Air 2s available for $319 including free shipping. A standard Apple one-year is included. Their price is $60 off original MSRP for this model. Read more
Mac Pros on sale for $200 off MSRP, refurbish...
B&H Photo has Mac Pros on sale for $200 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2799, $200 off MSRP - 3.5GHz 6-core Mac Pro: $3799, $200... Read more

Jobs Board

*Apple* Premier Retailer - Service Technicia...
DescriptionSimply Mac is the largest premier retailer for Apple products and solutions. At Simply Mac we are all Apple , all the time. Same products. Same prices. Read more
*Apple* Premier Retailer - Service Manager -...
DescriptionSimply Mac is the largest premier retailer for Apple products and solutions. At Simply Mac we are all Apple , all the time. Same products. Same prices. Read more
*Apple* Retail - Multiple Positions- Crows N...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Los Angeles, CA Introduction: We have immediate job openings for several Desktop Support Technicians with one of our Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.